Synchronized tunable mode-locked lasers

ABSTRACT

A synchronized laser system for illuminating a sample with first and second laser light pulses, said system comprising: a trigger, said trigger being operative to issue first and second trigger signals, said first and second trigger signals being emitted at an adjustable frequency with a predetermined delay therebetween; a first tunable mode-locked laser operative for emitting said first laser light pulses in response to receiving a train of said first trigger signals, a first wavelength of said first laser light pulses being dependent on said adjustable frequency in accordance with a first wavelength-frequency relationship; a second tunable mode-locked laser operative for emitting said second laser light pulses in response to receiving a train of said second trigger signals, a second wavelength of said second laser light pulses being dependent on said adjustable frequency in accordance with a second wavelength-frequency relationship; wherein said predetermined delay is such that said first and second laser light pulses are emitted so as to arrive substantially simultaneously in said sample; and said first and second wavelength-frequency relationships are selected to result in a predetermined relationship between said first and second wavelengths at each frequency.

FIELD OF THE INVENTION

The present invention relates to the general field of optics, and isparticularly concerned with the synchronization of tunable mode-lockedlasers.

BACKGROUND

“Electrical Wavelength-Tunable Actively Mode-Locked Fiber Ring Laserwith a Linearly Chirped Fiber Bragg Grating” by Li et Chan published inIEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 6, June 1998 799describes a tunable mode locked laser in which changes in the frequencyof variations in absorption in the laser cavity result in changes incavity length. In turn, changes in cavity length result in changes inthe wavelength of the light emitted by the laser. One manner ofachieving this result is to reflect the light in the cavity with achirped fiber Bragg grating (FBG) in which different wavelengths arereflected at different longitudinal positions along the FBG.

In some applications, there is a need to synchronize two or more suchlasers. More generally, there is a need to synchronize two or moredispersion-tuned actively mode-locked lasers (DTAML). In these lasers,dispersion is used to tune the mode-locked lasers. The use of FBGs is anexample of such dispersion tuning, but any other manner of creating adifferent round trip travel time of a laser pulse in a laser cavity as afunction of wavelength through the use of dispersion is usable. Thereare several issues in synchronization of such laser oscillators. The twomain issues are 1) adjusting the delay between the outputs of bothlasers so their pulses overlap and 2) having both lasers operate atexactly the same repetition rate. Since DTAML are actively model-locked,the timing of the output pulses is determined by the electronic signalsdriving the mode locker.

Against this background, there exists a need in the industry to providesynchronize tunable model-locked lasers. An object of the presentinvention is therefore to provide such lasers.

SUMMARY OF THE INVENTION

In a broad aspect, the invention provides a synchronized laser systemfor illuminating a sample with first and second laser light pulseshaving respectively first and second wavelengths, the system comprising:a trigger, the trigger being operative to issue first and second triggersignals, the first and second trigger signals being periodic and emittedat a common adjustable frequency with a predetermined delaytherebetween, the adjustable frequency being included in a predeterminedfrequency interval; a first tunable mode-locked laser operative foremitting the first laser light pulses in response to receiving a firsttrain of the first trigger signals, the first wavelength of the firstlaser light pulses being dependent on the adjustable frequency inaccordance with a first wavelength-frequency relationship, the firsttunable mode-locked laser being operative over a first repetition raterange part of the predetermined frequency interval to produce the firstlaser light with the first wavelength within a first laser tuning range;a second tunable mode-locked laser operative for emitting the secondlaser light pulses in response to receiving a second train of the secondtrigger signals, the second wavelength of the second laser light pulsesbeing dependent on the adjustable frequency in accordance with a secondwavelength-frequency relationship, the second tunable mode-locked laserbeing operative over a second repetition rate range part of thepredetermined frequency interval to produce the second laser light withthe second wavelength within a second laser tuning range. Thepredetermined delay is such that the first and second laser light pulsesare emitted so as to arrive substantially simultaneously in the sample.The first and second wavelength-frequency relationships are selected toresult in a predetermined relationship between the first and secondwavelengths at each adjustable frequency from the predeterminedfrequency interval at which the first and second repetition rate rangesoverlap.

In some embodiments of the invention, the first and secondwavelength-frequency relationships are such that the second wavelengthvaries less as a function of the adjustable frequency than the firstwavelength over the predetermined frequency interval so that over thepredetermined frequency interval, the first wavelength varies more thanthe second wavelength.

In some embodiments of the invention, the second wavelength varies atleast 100 times more slowly than the first wavelength as as function ofthe adjustable frequency.

In some embodiments of the invention, the sample defines an interactionbandwidth of interest including wavelengths over which a predeterminedlight-matter interaction occurs, the second laser tuning range beingwithin the interaction bandwidth.

In some embodiments of the invention, the sample includes a non-linearmaterial, the predetermined light-matter interaction including anon-linear light-matter interaction occurring in the interactionbandwidth of interest.

In some embodiments of the invention, the non-linear material is afrequency doubling material. For example, the frequency doublingmaterial is a Lithium Niobate (LiNbO₃) crystal, a Barium Borate crystal(BaB₂O₄) or a Potassium Titanyl Phosphate crystal (KTiOPO₄).

In some embodiments of the invention, a non-linear material is insertedbetween the second tunable mode-locked laser and the sample.

In some embodiments of the invention, the non-linear material is afrequency-doubling material operative for producing light having a thirdwavelength that is half the first wavelength when illuminated with thefirst laser light pulses.

In some embodiments of the invention, the first repetition rate range isentirely included in the second repetition rate range.

In some embodiments of the invention, the first and secondwavelength-frequency relationships are such that the first and secondwavelengths are respectively a first and a second monotonous function ofthe adjustable frequency.

In some embodiments of the invention, the predetermined frequencyinterval defines a first interval region and a second interval region,the first and second wavelength-frequency relationships being such thatthe first and second wavelengths are respectively monotonous functionsof the adjustable frequency over each of the first and second intervalregions.

In some embodiments of the invention, the first tunable mode-lockedlaser is operative for emitting third laser light pulses in response toreceiving the first train of the first trigger signals, a thirdwavelength of the third laser light pulses being dependent on theadjustable frequency in accordance with a third wavelength-frequencyrelationship, the first tunable mode-locked laser being operative overthe first repetition rate range to also produce the third laser lightwith the third wavelength within a third laser tuning range.

In some embodiments of the invention, the second tunable mode-lockedlaser is operative for emitting fourth laser light pulses in response toreceiving the second train of the second trigger signals, a fourthwavelength of the fourth laser light pulses being dependent on theadjustable frequency in accordance with a fourth wavelength-frequencyrelationship, the second tunable mode-locked laser being operative overthe second repetition rate range to also produce the fourth laser lightwith the fourth wavelength within a fourth laser tuning range. In someembodiments of the invention, outside of the first and second repetitionrate ranges, the first and second tunable mode-locked lasers areinoperational to produce respectively the first and second laser lightpulses.

Advantageously, the proposed system allows for substantiallysimultaneous emission of laser light pulses from two or moredispersion-tuned actively mode-locked lasers with precise control of theemitted wavelengths.

Other objects, advantages and features of the present invention willbecome more apparent upon reading of the following non-restrictivedescription of preferred embodiments thereof, given by way of exampleonly with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1, in a schematic view, illustrates a tunable laser usable with anembodiment of the present invention;

FIG. 2, in a schematic view, illustrates a tunable laser usable with analternative embodiment of the present invention;

FIG. 3, in a schematic view, illustrates a tunable laser usable withanother alternative embodiment of the present invention;

FIG. 4, in a schematic view, illustrates a tunable laser usable with yetanother alternative embodiment of the present invention;

FIG. 5, in a schematic view, illustrates a tunable laser usable with yetanother alternative embodiment of the present invention;

FIG. 6, in a schematic view, illustrates a tunable laser usable with yetanother alternative embodiment of the present invention;

FIG. 7, in a schematic view, illustrates a tunable laser usable with yetanother alternative embodiment of the present invention;

FIG. 8, in a schematic view, illustrates an apparatus usable forarbitrarily shaping the spectrum of a pulse of light;

FIG. 9, in a schematic view, illustrates a tunable laser usable with yetanother alternative embodiment of the present invention;

FIG. 10, in a schematic view, illustrates a tunable laser usable withyet another alternative embodiment of the present invention;

FIG. 11, in a schematic view, illustrates a synchronized laser system inaccordance with an embodiment of the present invention;

FIG. 12, in a schematic view, illustrates a synchronized laser system inaccordance with an alternative embodiment of the present invention;

FIG. 13, in a schematic view, illustrates a synchronized laser system inaccordance with another alternative embodiment of the present invention;and

FIG. 14, in a schematic view, illustrates a synchronized laser system inaccordance with yet another alternative embodiment of the presentinvention.

DETAILED DESCRIPTION

FIGS. 1 to 10 describe various tunable mode-locked lasers that one maywish to synchronize with other similar lasers. Referring to FIG. 1,there is shown a tunable laser 10 for selectively emitting laser light12 having a first wavelength and a second wavelength. While in someembodiments of the invention the tunable laser 10 is able emit laserlight 12 having two different, discretely spaced apart, wavelengths, itis also within the scope of the invention to have a tunable laser 10that is able to emit laser light 12 having more than two differentwavelengths and laser light having a wavelength contained within asubstantially continuous spectrum of wavelengths included in apredetermined wavelength interval.

The tunable laser 10 includes a pump light source 14 for emitting a pumplight (not shown in the drawings). The tunable laser 10 also includes anoptical resonator 16. The optical resonator 16 has a configuration,optical properties and dimensions such that a first round trip time ofthe laser light 12 having the first wavelength in the optical resonator16 differs from a second round trip time of the laser light 12 havingthe second wavelength in the optical resonator 16. A gain medium 18 isinserted in the optical resonator 16 and is optically coupled to thepump light source 14. The gain medium 18 defines a gain medium first end26 and a substantially opposed gain medium second end 28. The readerskilled in the art will understand that the terminology “gain mediumfirst and second ends 26 and 28” does not imply that the gain medium 18is necessarily rectilinear. For example, the gain medium may be formedby a rolled optical fiber. The gain medium 18 is responsive to the pumplight for converting the pump light into the laser light 12. Forexample, the tunable laser 10 includes a pump light input port 20, alsoreferred to as a pump coupler, optically coupled to the gain medium 18for receiving the pump light and conveying the pump light to the gainmedium 18. It should be noted that in alternative embodiments of theinvention, any other suitable gain medium 18 is usable, including forexample a semi-conductor gain medium, among other possibilities.

An optical intensity modulator 22 is inserted in the optical resonator16 for selectively absorbing a portion of the laser light 12 as thelaser light 12 propagates back and forth in the optical resonator 16.The optical intensity modulator 22 has a light absorption coefficientthat is modulated with a modulation period. The modulation period isselectively adjustable between a first modulation period value and asecond modulation period value. The first and second round trip timesare substantially equal to a respective integer multiple of respectivelythe first and second modulation period values.

The tunable laser 10 further includes an output port 24 for releasingthe laser light 12 from the optical resonator 16. In some embodiments ofthe invention, a tunable laser cavity is provided instead of a tunablelaser 10. The tunable laser cavity is simply a tunable laser 10 fromwhich the pump light source 14 has been removed. The tunable lasercavity is usable with the pump light source 14 to build the tunablelaser 10.

When the gain medium 18 is pumped with the pump light, modulating theoptical intensity modulator 22 with the first modulation period valueproduces laser light having the first wavelength. Also, modulating theoptical intensity modulator 22 with the second modulation period valueproduces laser light having the second wavelength.

Indeed, when the optical intensity modulator 22 is modulated, there willbe periodic time intervals during which the optical intensity modulator22 absorbs more light than at subsequent or previous moments. Since thelaser light 12 is preferentially transmitted through the opticalintensity modulator 22 at predetermined periodic time intervals, therewill be a preference for the tunable laser 10 to operate with laserlight pulses circulating within the optical resonator 16 in a mannersuch that these laser light pulses have a round trip time that is equalto the modulation period with which the optical intensity modulator ismodulated, or a multiple of this modulation period. The configuration ofthe optical resonator 16 will therefore favor laser light pulses withinthe optical resonator 16 that have a round trip time correspondingrespectively to the first and second wavelengths when the opticalintensity modulator 22 is respectively modulated with the first andsecond modulation period values. The pulse duration of the pulses isgoverned by many factors, among which are the power provided by the pumplight source 14, the dispersion in the whole tunable laser 10 and theexact wave shape of the modulation provided by the optical intensitymodulator 22.

It has been found particularly advantageous in some embodiments of theinvention to change the modulation period in steps instead ofcontinuously. Indeed, lasers have a tendency to be locked at apredetermined wavelength when operating. Changing the modulation periodin a substantially continuous manner from the first to the secondmodulation period values may then cause instabilities and, in turn,promote difficult mode locking at the second wavelength. By changing themodulation periods in discreet steps, tuning occurs faster usingcommonly available components.

In some embodiments of the invention, modulating the absorptioncoefficient of the optical intensity modulator 22 with a signal that isthe sum of many single-frequency signals helps in modulating the outputlaser light 12. For example, by modulating the absorption coefficientwith a signal that is the sum of two sinusoidal signals havingfrequencies that are close to each other, the laser light 12 has anoutput that varies sinusoidally with a frequency that is equal to thebeat frequency of the two sinusoidal signals.

In other embodiments, the optical intensity modulator has an absorptioncoefficient that is modulated temporally in any suitable manner toproduce a corresponding temporal intensity profile of the pulses oflaser light produced. The temporal modulation is periodic with a periodcorresponding to the round trip time in the optical resonator 16.

In yet other embodiments, multi-wavelength pulses can be produced. Tothat effect, the optical resonator 16 is such that the two or morewavelengths have the same round trip time in the optical resonator 16.In the specific case in which the optical resonator 16 includes a fiberBragg grating, which are described in further details hereinbelow, thegroup delay as a function of wavelength in the Bragg grating must notincrease of decrease monotonically. In a specific example ofimplementation, the number of wavelengths is equal to the number ofzeros in the dispersion. Also, the derivative of variations in reflectedwavelength with position along the Bragg grating determines the tuningrate of each of the wavelengths. However, there are other manners inwhich the optical resonator 16 can be configured to achieve the sameresults. It should be noted that due to the physical structure of theseembodiments, laser light at all wavelengths are produced synchronouslyand are tuned at different, customizable rates.

In yet other embodiments of the invention, the optical resonator 16 issuch that different wavelength light impulsions have different roundtrip times, as in the “base” embodiment first described hereinabove. Inthese embodiments, by modulating the optical intensity modulator 22 witha modulation that is a sum of modulation corresponding to manywavelengths produced in the case of single period modulation, pulsetrains of laser light having impulsions of different wavelengths can beproduced. In other words, the pulse trains produced are a linearsuperposition of individual pulse trains having different wavelengths,and therefore repetition rates. This increases the pulse repetition rateand scans many frequencies over relatively short amounts of time. As inthe other embodiments of the invention, the optical modulation is eithercomplete (complete absorption) or partial (partial absorption at maximalabsorption), with a possibility of pulse shaping.

In the embodiment of the invention shown in FIG. 1, the opticalresonator 16 includes a first reflector 30 and a second reflector 32.The first and second reflectors 30 and 32 are each reflective at aboutthe first and second wavelengths. The first and second reflectors 30 and32 are optically coupled to the gain medium 18 respectively through thegain medium first and second ends 26 and 28. It should be noted that, insome embodiments of the invention, other optical components are presentbetween either of the reflectors 30 and 32 and the gain medium 18.Therefore, the first and second reflectors 30 and 32 need not bedirectly physically coupled to the gain medium 18.

The first reflector 30 includes a first reflector first portion 34 and afirst reflector second portion 36 for reflecting respectively the laserlight 12 having the first and second wavelengths. The first reflectorfirst and second portions 34 and 36 are respectively spaced apart fromthe gain medium first end 26 by a first reflector first portion-to-gainmedium distance and a first reflector second portion-to-gain mediumdistance. The first reflector first portion-to-gain medium distance issmaller than the first reflector second portion-to-gain medium distance.Therefore, the first reflector first portion 34 must be transmitting thesecond wavelength so that laser light having the second wavelength canreach the first reflector second portion 36.

Similarly, the second reflector 32 includes a second reflector firstportion 38 and a second reflector second portion 40 for reflectingrespectively the laser light 12 having the first and second wavelengths.The second reflector first and second portions 38 and 40 arerespectively spaced apart from the gain medium second end 28 by a secondreflector first portion-to-gain medium distance and a second reflectorsecond portion-to-gain medium distance. The second reflector firstportion-to-gain medium distance is smaller than the second reflectorsecond portion-to-gain medium distance. Therefore, the second reflectorfirst portion 38 must be transmitting the second wavelength so thatlaser light having the second wavelength can reach the second reflectorsecond portion 40.

The first and second reflector first portions 34 and 38 are thusdistanced from each other by a smaller distance than the first andsecond reflector second portions 36 and 40. This will cause the laserlight 12 having the first wavelength to have a smaller first round triptime between the first and second reflector first portions 34 and 38than the round trip time of the laser light 12 having the secondwavelength between the first and second reflector second portions 36 and40. In a specific embodiment of the invention, the first and secondreflectors 30 and 32 each include a respective fiber Bragg grating. Forexample, the first reflector first and second portions 34 and 36 includerespectively a first and a second fiber Bragg grating segment. In aspecific embodiment of the invention, the first reflector first andsecond portions 34 and 36 each include a respective chirped fiber Bragggrating segment, which may be formed by having a single chirped fiberBragg grating that defines both the first reflector first and secondportions 34 and 36. In some specific embodiment of this latterconstruction, a tunable laser 10 having continuous wavelength selectionis provided.

In some embodiments of the invention, the first wavelength is largerthan the second wavelength and the chirped fiber Bragg gratings includedin the first and second reflectors 30 and 32 also provides dispersioncompensation. However, in alternative embodiments of the invention, thefirst wavelength is smaller than the second wavelength, which may beuseful in embodiments in which other components of the proposed tunablelaser 10 have anomalous dispersion properties.

In some embodiments of the invention, the first reflector 30 includes arelatively highly reflective chirped fiber Bragg grating. In theseembodiments, substantially all the light incoming at the first reflector30 is reflected back towards the gain medium 18. The second reflector 32is an output chirped fiber Bragg grating and is not perfectly reflectiveso that some of the laser light 12 can be transmitted through the secondreflector 32, which therefore provides the output port 24. Other mannersof outputting the laser light 12 from the tunable laser 10 are withinthe scope of the invention and some of them are described in furtherdetails hereinbelow.

In some embodiments of the invention, the gain medium 18 has a firstgain at the first wavelength and a second gain at the second wavelength.The first and second gains differ from each other. In these embodiments,to facilitate the production of laser light 12 having substantiallysimilar powers at the two wavelengths, the first and second fiber Bragggrating segments included in the first reflector first and secondportions 34 and 36 have respectively a first segment reflectivity and asecond segment reflectivity. The first and second segment reflectivitiesare such that the tunable laser 10 has substantially constant gain atthe first and second wavelengths. Therefore, it is possible to selectthe reflectivity of the first reflector first and second portions 34 and36 so that the reflection of the laser light 12 at these first reflectorfirst and second portions 34 and 36 compensates for the non-flat gaincurve of the gain medium 18.

In some embodiments of the invention, one or both the first and secondreflectors 30 and 32 are each made from a chirped fiber Bragg gratingdefining a variable group delay therealong. Therefore, the fiber Bragggrating segments included in different portions along the first andsecond reflectors 30 and 32 have different group delay characteristics,which affects the duration of laser light pulses produced using thetunable laser 10. Also, pulse characteristics other than the duration ofthe laser light pulses can be modified by selecting suitable groupdelays for fiber Bragg grating segments included in the first reflectorfirst and second portions 34 and 36.

In yet other embodiments of the invention, the position of the secondreflector first and second portions 38 and 40 is reversed with respectto the gain medium 18 while the first reflector first and secondportions 34 and 36 remain in the same position. In these embodiments,wavelength selection of the laser light 12 is permitted by spacing apartthe second reflector first and second portions 38 and 40 from each otherby a greater distance than the distance by which the first reflectorfirst and second portions 34 and 36 are spaced apart from each other.When chirped fiber Bragg gratings are used in the first and secondreflectors 30 and 32, different group delay slopes are produced, and thelaser operates with a group velocity dispersion that is the differencebetween the two group delay slopes. A mix between a soliton laser and anormal dispersion laser is thus formed that reduces or eliminatesKelly's side bends.

In some embodiments of the invention, the gain medium 18 includes adoped gain fiber. Such doped gain fibers are well known in the art andwill therefore not be described in further details. It is also withinthe scope of the invention to manufacture tunable lasers 10 using anyother suitable gain medium 18. Also, the pump light source 14 is anysuitable pump light source 14 that can emit pump light that allows thegain medium 18 to produce the laser light 12 having both the first andsecond wavelengths. For example, the pump light source 14 includes alight emitting diode.

The pump light input port 20 is also any suitable pump light input port20. For example, the pump light input port 20 includes a wave divisionmultiplexer (WDM) that allows light having the first and secondwavelengths to be freely transmitted, or substantially freelytransmitted therethrough but which, through optical isolators or anyother suitable means, substantially prevents light, and especially thelaser light 12, from being transmitted back towards the pump lightsource 14. The WDM also allows for receiving pump light emitted by thepump light source 14 and transmitting this pump light into the opticalresonator 16 and, more specifically, into the gain medium 18.

The optical intensity modulator 22 is any suitable component allowingvariations in the transmission of the laser light 12 having the firstand second wavelengths therethrough. Typically, the optical intensitymodulator 22 takes the form of a component that is coupled to andinserted between the chirped fiber Bragg grating forming the firstreflector 30 and the WDM multiplexer forming the pump light input port20. However, any other physical configurations of the optical intensitymodulator 22 are within the scope of the invention. In some embodimentsof the invention, the optical intensity modulator 22 includes anelectro-optic modulator.

In some embodiments of the invention, the optical intensity modulator 22is a component that allows the laser light 12 to pass therethrough withtwo different absorption levels. For example, one level allowssubstantially all the light incoming at the optical intensity modulator22 to pass therethrough. At a second level, a predetermined fraction ofthe light incoming at the optical intensity modulator 22 is nottransmitted. In these embodiments, periodically changing the absorptioncoefficient of the optical intensity modulator 22 between the firstlevel and the second level preferentially selects a laser light pulsethat travels through the optical resonator 16 as described hereinabove.

The reader skilled in the art will readily appreciate that the opticalintensity modulator 22 need not absorb all or a large fraction of thelight circulating within the tunable laser 10 for the mode lockingeffect provided by the optical intensity modulator 22 to be provided.Indeed, only relatively small variations in the absorption coefficientare sufficient in some embodiments of the invention to produce thedesired effect.

In some embodiments of the invention, the optical intensity modulator 22includes a variable attenuation modulator. In opposition to thepreviously described optical intensity modulator, the variableattenuation modulator allows for a substantially continuous variation inthe absorption coefficient of the optical intensity modulator 22 over apredetermined absorption range. In this embodiment, the power of thelaser light 12 may therefore be regulated using the optical intensitymodulator 22, in addition to being regulated using the power of the pumplight source 14.

As illustrated in FIG. 1, in some embodiments of the invention, thetunable laser 10 includes a controller 42 for controlling the modulationperiod of the optical intensity modulator 22. Typically, fiber Bragggratings, such as the fiber Bragg gratings usable in the first andsecond reflectors 30 and 32, do not have ideal, theoreticallypredictable, reflection spectra at different locations therealong due tomanufacturing defects. However, once a fiber Bragg grating has beencharacterized, it is possible to map the distance from one end of thefiber Bragg grating to each location therealong and to associate witheach of these locations a specific wavelength that is reflected.Therefore, by characterizing the first and second reflectors 30 and 32in this manner, the controller 42 can be programmed to select preciselythe wavelength at which the laser light 12 will be emitted for aspecific tunable laser 10 as the modulation period can then be selectedto achieve this wavelength.

Also, the group delay characteristics of chirped fiber Bragg gratingsare not perfect. Imperfections create a group delay ripple which maycause unwanted effects in the pulsed laser light 12. A ratio between thegroup delay ripple and the group delay as a function of frequency isherein referred to as the ratio spectrum. The pulses of laser light 12produced by the tunable laser 10 have a pulse spectrum. It is preferableto manufacture the chirped Bragg gratings with sufficient precision thatthe ratio spectrum is substantially disjoint from the pulse spectrum. Inother words, regions of the ratio spectrum in which there is asignificant power should be separate from regions of pulse spectrum atwhich there is a significant power.

FIG. 2 illustrates an alternative tunable laser 10A. The tunable laser10A has many components that are substantially similar to those of thetunable laser 10. These components will therefore not be described infurther details.

As seen from FIG. 2, the tunable laser 10A differs from the tunablelaser 10 in that it includes two pump light sources 14, each coupled tothe gain medium 18 through a respective pump light input port 20. Also,it is within the scope of the invention to have more than two pump lightsources 14. In addition, a tap 44 is provided, for example at an end ofthe first reflector 30 opposed to the gain medium 18, so that aphotodiode 46, or any other suitable light intensity measurement device,can be used to measure the taped light and feed this light intensity tothe controller 42. The controller 42 is therefore connected to thephotodiode 46 so that light intensity measurements can be transmitted bythe photodiode 46 to the controller 42. The controller 42 is alsooperatively coupled to the pump light source 14 for controlling theintensity of the pump light. The controller 42 is then used in afeedback loop to control the power provided by the pump light sources 14in response to light intensity measurements to obtain a predeterminedpower for the laser light 12.

FIG. 3 illustrates yet another tunable laser 10B. The tunable laser 10Bbeing similar also to the tunable laser 10. A difference that occurs inthe tunable laser 10B is that an alternative first reflector 30B isused. The alternative first reflector 30B is such that the firstreflector 30B reflects the laser light 12 having both the first andsecond wavelengths at substantially similar first reflector-to-modulatordistances from the optical resonator 16. For example, this is achievedby using a mirror instead of a fiber Bragg grating in the firstreflector 30B. In yet other embodiments of the invention, the firstreflector 30B is replaced by an optical circulator or a loop of opticalfiber that returns all incoming light toward the second reflector 32.

FIG. 4 illustrates yet another tunable laser 10C in which the opticalresonator 16, the gain medium 18 and optical intensity modulator 22 areall polarization maintaining, as illustrated by the dashedrepresentation of these components. Therefore, the tunable laser 10C isable to produce polarized laser light 12. To select the polarization, apolarizer 47 is inserted in the optical resonator 16C for polarizing thelaser light 12.

FIG. 5 illustrates yet another tunable laser 10D. The tunable laser 10Duses only a single reflector 30 instead of the first and secondreflectors 30 and 32. The resonance in the optical resonator 16D isprovided by using an optical circulator 48. The optical circulator 48includes a circulator first port 50, a circulator second port 52 and acirculator third port 54. The optical circulator 48 is configured in amanner such that the laser light incoming at the circulator first port50 is emitted at the circulator second port 52, laser light incoming atthe circulator second port 52 is emitted at the circulator third port 54and laser light incoming at the circulator third port 54 is emitted atthe circulator first port 50. The circulator first port 50 is opticallycoupled to the gain medium 18 through the gain medium first end 26 withthe optical intensity modulator 22 inserted between the gain mediumfirst end 26 and the circulator first port 50. The circulator secondport 52 is optically coupled to the reflector 30 and the circulatorthird port 54 is optically coupled to the gain medium 18 through thegain medium second end 28 with the pump light input port 20 insertedbetween the gain medium second end 28 and the circulator third port 54.The reflector 30 is a reflector similar to the first and secondreflectors 30 and 32 and has a structure and a function substantiallysimilar to that of the first and second reflectors 30 and 32. In thisembodiment, the reflector 30 allows for the emission of the laser light12 by the laser 10D by letting a portion of the laser light 12 to betransmitted through the reflector 30. In this embodiment of theinvention, a unidirectional loop is created, which reduces losses in thetunable laser 10D caused by the optical intensity modulator 22.

In yet another embodiment of the invention, a tunable laser 10E shown inFIG. 6 is provided. The tunable laser 10E includes an alternative outputport 24E inserted between the optical intensity modulator 22 and thecirculator first port 50. The tunable laser 10E has a configurationsubstantially similar to the configuration of the tunable laser 10D,with the exception that the reflector 30 is highly reflective and,therefore, does not allow for laser light 12 to be transmittedtherethrough. Instead, an output port 24E in the form of a fiber coupleror, in other words, a tap, is provided for tapping into the tunablelaser 10E and therefore releasing the tunable laser light 12.

FIG. 7 illustrates yet another geometry for a tunable laser 10F in whichthe second reflector 32F includes an optical circulator 48 for receivingthe laser light 12 from the gain medium 18 and returning the laser lightback 12 to the gain medium 18. In this embodiment, the circulator firstport 50 is optically coupled to the circulator third port 54 with anoptical switch 51 and the optical intensity modulator 22 insertedtherebetween. The circulator second port 52 is optically coupled to thefirst reflector 30 with the gain medium 18 and the pump light input port20 inserted therebetween. Advantageously, various optical components canbe inserted in the loop formed between the circulator first and thirdports 50 and 54 to allow emission of the laser light 12, modulation ofthe intensity of the laser light 12 and any other conditioning orcharacterization of the laser light 12.

The optical switch 51 defines the output port 24F and is usable forselectively releasing the laser light 12 from the optical resonator 16Fand confining the laser light 12 in the optical resonator 16F. Morespecifically, in one state of the optical switch 51, all the lightincoming at the switch 51 is fed back into the optical resonator 16F.This allows for build up of laser light power inside the opticalresonator 16F. When a pulse is to be let out of the optical resonator16F, the switch 51 is switched to the other state in which a part or allof the light incoming at the switch 51 is output at another port thatforms the output port 24F.

The reader skilled in the art will readily appreciate that inalternative embodiments of the invention, the tunable lasers 10D, 10Eand 10F have an optical intensity modulator 22 that is located at anysuitable location between the circulator first and third ports 50 and54. In other words, the exact position of the optical intensitymodulator 22 in the loop formed between the circulator first and thirdports 50 and 54 can be varied along this loop while achieving tunablelasers that perform satisfactorily. In yet other embodiments of theinvention, the various components of the tunable lasers 10D, 10E and 10Fare configured in any suitable order as long as an optical resonator isformed. Also, the pump light can travel either in the direction of thelaser light or in the opposite direction.

Referring to FIG. 8, there is shown an apparatus 100 for arbitrarilyshaping the spectrum of a pulse of light. In some embodiments of theinvention, the apparatus 100 is inserted at a suitable location in thetunable lasers 10 to 10F to provide laser light pulses having anarbitrary spectrum.

The apparatus 100 includes an optical circulator 48. The opticalcirculator 48 includes a circulator first port 50, a circulator secondport 52 and a circulator third port 54. The optical circulator 48 isconfigured in a manner such that light incoming at the circulator firstport 50 is emitted at the circulator second port 52, and light incomingat the circulator second port 52 is emitted at the circulator third port54. The circulator first port 50 is optically coupled to an input port102 used for receiving laser light. The circulator second port 52 isoptically coupled to a reflector 30, such as a chirped Bragg grating,that reflects light having different wavelengths at different locationstherealong, and the circulator third port 54 is optically coupled to anoptical modulator 22. The optical modulator 22 is optically coupled toan output port 24 usable for releasing the light modulated by theoptical modulator 22.

The reflector 30 spreads temporally the different frequencies comprisedin the light incoming at the input port 102. Therefore, by suitablemodulation of the absorbance of the optical modulator 22 as a functionof time, light having any desired spectrum is achievable. Also, eachsuccessive pulse of light incoming at the input port 102 can be shapeddifferently. In some embodiments of the invention, the output port 24 isoptically coupled to an optical component having a dispersion inversethat of the reflector 30, thereby temporally compressing the spectrallyshaped pulse.

It should be noted that in alternative embodiments of the invention, anycomponent that spreads temporally light having different frequencies isusable instead of the reflector 30.

In yet another embodiment of the invention, a tunable laser 10G shown inFIG. 9 is provided. The tunable laser 10G is similar to the tunablelaser 10E shown in FIG. 6. However, a delay element 60 is inserted atany suitable location in the resonator of the laser 10G for delaying, orchanging the phase, of the light that passes through the delay element60. For example, the delay element 60 is inserted between the pumpcoupler 20 and the gain medium 18. The delay element 60 changes thepulse frequency/light wavelength relationship of the tunable laser 10G.If the delay element 60 is a variable delay element, this relationshipcan be adjusted to any needed relationship. For example, the delay couldbe selected so that all wavelengths emitted by the tunable laser 10 Ghave the same pulse frequency.

In yet another embodiment of the invention, a tunable laser 10H shown inFIG. 10 is provided. The tunable laser 10H is similar to the tunablelaser 10E shown in FIG. 6. However, many reflectors 30 are used inparallel, instead of a single one. Each of the reflectors 30 reflects adifferent wavelength for a given resonating cavity length. This createsa laser in which pulses of laser light including many differentwavelengths are created, all the wavelengths being synchronized. Whiletwo reflectors are shown in the drawings, any suitable number ofreflectors is usable. The reflectors 30 are coupled to the remainder ofthe tunable laser 10H using any suitable coupler 62, such as, forexample, a wavelength division multiplexer. In other similar embodimentsof the invention, the reflectors 30 reflecting different wavelengths areembodied in a single component, for example as superposed Bragg gratingsin a single optical fiber.

In yet another example of a multi-wavelength laser, a laser having astructure similar to the one of the lasers 10 or 10A to 10F producespulses including many wavelengths by having their reflectors (reflectingexclusively each light having a respective wavelength) create cavitiesof different lengths for different wavelengths, the lengths being suchthat round-trip travel times for all wavelengths are all integermultiples of a base period. By suitably selecting the frequency at whichthe optical intensity modulator 22 is modulated, simultaneous productionof light pulses at all the wavelengths is possible. For example, if acavity of a base length is created for a first wavelength and a cavityof double the base length is created for a second wavelength, modulatingthe optical intensity modulator at a speed required to create pulses atthe first wavelength automatically allows creation of pulses at thesecond wavelength.

By selecting appropriate wavelengths for pulses circulating in theoptical resonator and the delays (or phase difference) between them, itis possible to synthesise resulting total pulses having arbitraryshapes. In some embodiments of the invention, the resulting pulse isreflected by a chirped fiber Bragg grating prior to use to compress theresulting total pulse.

Another application of multi-wavelength lasers such as those describedhereinabove resides in the possibility to build up simultaneously withinthe optical resonator laser light having many different wavelengths.Since many wavelengths are present, switching the operation of the laserfrom one wavelength to the other is facilitated as stabilisation of thelaser for emission at each successive wavelength is already in partcompleted.

In the above-described tunable lasers 10 to 10H, using suitablecomponents allows for variations in the duration of the laser lightpulses by varying the intensity of these laser light pulses. In turn,this intensity is adjustable by varying many controllable variables,such as the duration and time evolution profile of the optical intensitymodulation provided by the optical intensity modulator 22 and the powerprovided by the pump light source 14. In some embodiments, the firstreflector 30, the second reflector 32 or both the first and secondreflectors 30 and 32 have an adjustable dispersion, which is then alsousable to change the laser light pulses shape and duration. It should benoted that the variations in the pulse duration is achievable withoutchanging the pulse repetition frequency.

While some embodiments of a tunable laser have been describedhereinabove, it is within the scope of the invention to have many othervariants. Also, it is within the scope of the invention to use manyconcepts associated with lasers to operate the proposed tunable lasersin different operation ranges. For example, the proposed laser may be Qswitched and, as described hereinabove in a specific embodiment, cavitydumping may be used.

FIGS. 11 to 14 illustrate synchronized laser systems in accordance withvarious embodiments of the present invention. With reference to FIG. 11,a first synchronized laser system 100 includes first and second tunablemode-locked lasers 102 and 104 and a trigger 110. Any suitable tunablemode-locked laser 102 and 104 is usable, for example the lasers 10 to10H described hereinabove, among other possibilities.

The trigger 110 is operative to issue first and second trigger signalsrespectively to the first and second tunable mode-locked lasers 102 and104, the first and second trigger signals being periodic and emitted ata common adjustable frequency with a predetermined delay therebetween.The adjustable frequency is included in a predetermined frequencyinterval. The predetermined delay may be frequency-dependent or fixedfor any system. The predetermined delay corrects, among other factors,for the variations in properties of the first and second tunablemode-locked lasers 102 and 104 between different synchronized lasersystems 100. The predetermined delay is such that the first and secondlaser light pulses are emitted so as to arrive substantiallysimultaneously in a sample 112. In other words, the predetermined delaymay also compensate in some embodiments for variations in propagationdelays between the first and second tunable mode-locked lasers 102 and104 and the sample to illuminate. Such delays may be caused by opticalfibers or any other optical components guiding the first and secondpulses to the sample 112, among other possibilities.

The delay between the first and second trigger signals may be created byduplicating an initial trigger and delaying the duplicated trigger inany suitable manner while not delaying the initial trigger. Such delaysare well-known in the art and manners of achieving them are not furtherdescribed.

The first and second tunable mode-locked lasers 102 and 104 areoperative for emitting respectively first and second laser light pulsesin response to receiving respectively a first and second trains of thefirst and second trigger signals. The first and second wavelengths ofrespectively the first and second laser light pulses are dependent onthe adjustable frequency of the first and second trigger signals inaccordance respectively with first and second wavelength-frequencyrelationships. The first and tunable mode-locked lasers 102 and 104 areoperative over respectively first and second repetition rate ranges partof the predetermined frequency interval to produce the first and secondlaser light with the first wavelength within a first laser tuning rangeand the second wavelength within a second laser tuning range. The firstand second wavelength-frequency relationships are selected to result ina predetermined relationship between the first and second wavelengths ateach adjustable frequency from the predetermined frequency interval atwhich the first and second repetition rate ranges overlap. Outside ofthe first and second repetition rate ranges, in some embodiments, thereis no laser light produced by the first and second tunable mode-lockedlasers 102 and 104.

For example, the first and second tunable mode-locked lasers 102 and 104are similar to the laser 10E described hereinabove. However, any othersuitable dispersion-tuned actively mode-locked lasers (DTAML) areusable. The remainder of this document, unless mentioned otherwise,refers to the specific case in which the first and second tunablemode-locked lasers 102 and 104 are similar to the laser 10E to simplifythe description of the system 100. In this case the first and secondtrigger signals are sent to the optical intensity modulator 22 totrigger the change in optical absorption. For brevity, the first andsecond unable mode-locked lasers 102 and 104 are referred hereinbelowsimply as lasers 102 and 104.

In FIG. 11, the first reflectors 106 and 108 of the first and secondlasers 102 and 104 are chirped fiber Bragg gratings, corresponding tothe reflector 30 of FIG. 6. The first and second reflectors 106 and 108are not drawn necessarily to scale, but longer elements in FIG. 11correspond to longer elements in the physical embodiment of the system100. Similarly, the spacing of the short vertical bars shown on each ofthe first and second reflectors 106 and 108, which represent fiber Bragggratings, reflects the physical embodiment of the system 100, once againnot to scale, with closer bars representing portions of the first andsecond reflectors 106 and 108 that reflect shorter wavelengths.

For both the first and second lasers 102 and 104 to have the samerepetition rate, they need to have the same cavity length or the cavitylengths need to be integer multiples of each other. Cavity length isindicated by travel time of light in the lasers. In other words, incases in which optical fibres are used, the physical dimension of thecavity is not the important factor, but the travel time in the cavity isthe critical factor. Indeed, depending on the type of fiber used in eachof the first and second lasers 102 and 104, and of the wavelength of thelight circulating therein, the cavities may have require differentphysical dimensions to achieve a given propagation time over the cavity.

A particularity of DTAML is that they operate over a range of repetitionrates because their laser cavity is dispersive (that is, the differentwavelengths have different propagation times in the cavity). Let us namethat range of repetition rates R₁ for laser 102, which corresponds tothe first repetition range, and R₂ for laser 102, which corresponds tothe second repetition range (each range R_(x) starts at repetition ratef₀ and ends at f_(n) which correspond respectively to wavelengths of thelaser light pulses λ₀ and λ_(v)). So to synchronize two DTAMLs, we needto overlap their repetition rate ranges. If the ranges partiallyoverlaps, we can only partly tune both lasers over the common repetitionrates—outside of this zone, only one (or no) laser will operate. One wayto be sure that one laser (for example laser 102) operates over itswhole tuning range without the stringent condition of equalizing exactlyboth cavity lengths, is to make sure that its repetition rate range issmaller than the repetition rate range of the other laser (in thisexample, laser 104) (R₁<R₂) as shown in FIG. 11. When synchronizing twoDTAMLs, the laser 102 that tunes over its whole tuning range must have asmaller repetition rate range than the other laser(s). This accomplishby having the product of the magnitude of the net dispersion(|D/)×tuning range (Δλ) of laser 102 be smaller than that of laser 104(|D₁/·Δλ₁</D₂/·Δλ₂). More generally, it is advantageous in someembodiments to have the second wavelength-frequency relationships suchthat the first wavelength varies less as a function of the adjustablefrequency than the second wavelength over the predetermined frequencyinterval so that over the predetermined frequency interval, the secondwavelength varies more than the first wavelength.

When we change the repetition rate, both DTAMLs will tune theirfrequency simultaneously. An interesting case is to have one of theDTAML operate at a quasi-fixed wavelength while the other laser'swavelength is tuning. For example, the second wavelength varies at least100 times more slowly than the first wavelength as as function of theadjustable frequency. Therefore, in this case, the relationship betweenthe first and second wavelengths is that the second wavelength remainssubstantially constant when the first wavelength is changed. If we stillwant to tune laser 102 over its whole tuning range, let us have laser104 operating at a quasi-fixed wavelength. In this case, the overlapcondition (|D₁/·Δλ₁</D₂/·Δλ₂) still applies. To have laser 104quasi-fixed, we limit its tuning range (Δλ₂<<Δλ₁). To satisfy theoverlap condition, we must have (|D₂/>>|D₁/). To synchronize two DTAMLsand have one of those lasers operate at a quasi-fixed wavelengthregardless of the repetition rate, the dispersion of this laser must bevery high.

This asymmetry in the rates of variation of wavelength as a function ofthe repetition rate of the trigger 110 is useful in many situations. Forexample, the sample 112 defines an interaction bandwidth of interestincluding wavelengths over which a predetermined light-matterinteraction occurs, the second laser tuning range being within theinteraction bandwidth. In a specific example, the predeterminedlight-matter interaction including a non-linear light-matter interactionoccurring in the interaction bandwidth of interest. For instance, twobroad classes of material with such nonlinear interactions are nonlinearcrystals and molecular nonlinearity. Nonlinear crystals presenting X⁽²⁾nonlinearity will convert the wavelengths from the first laser 102 andthe second laser 104 through sum-frequency generation or differencedifference frequency generation to a third wavelength that isrespectively lower or higher than the wavelengths of both lasers 102 and104. Among such crystals are AgGaSe₂, AgGaS₂, GaAs and GaSe crystals. Avery specific example of a non-linear material would be a frequencydoubling material, such as Lithium Niobate crystals (LiNbO₃), BariumBorate crystals (BaB₂O₄) or Potassium Titanyl Phosphate crystals(KtiOPO₄). Such frequency-doubling materials are operative for producinglight having a third wavelength that is half the second wavelength whenilluminated with the second laser light pulses. Many such materials havea relatively small wavelength bandwidth over which frequency doublingoccurs, for example a few nanometers, and in some cases as small as onenanometer. Another other class of material is any molecular substancehaving a X⁽³⁾ Raman gain spectrum. The wavelengths of both lasers 102and 104 will generate a third wavelength if their difference (in energy)matches the gain. Most gases and organic compounds have distinctiveRaman spectra. The present invention is well suited to CoherentAntistokes Raman Spectroscopy (CARS) in which molecular spectra areobtained when two laser pulses having different wavelengths interact ina material, with the difference between the wavelength being varied. Forexample, this may be performed by keeping the second wavelength of thesecond laser 104 fixed, or almost fixed, and varying the firstwavelength of a first laser 102. In another example (not shown in thedrawings), instead of having the sample including the non-linearmaterial, the non-linear material is inserted between the second laser104 and the sample 112.

As seen in FIG. 11, typically the first and second wavelength-frequencyrelationships are such that the first and second wavelengths arerespectively a first and a second monotonous function of the adjustablefrequency. A monotonous function is such that as the independentvariable increases, the dependent variable never decreases or neverincreases.

We have assumed thus far that both lasers 102 and 104 had one region intheir respective first reflectors 106 and 108 where the reflectedwavelength changes with repetition rate. However, as seen in FIG. 12 forthe system 100A, each laser 102A and 104A can have multiplelongitudinally spaced apart zones in their respective first reflectors106A and 108A where the wavelengths changes with repetition rates(corresponding to various repetition rate ranges R_(m)), whetherdiscrete and separated or being contiguous and continuous. Therefore,different reflector regions are used at different regions of thepredetermined frequency interval. It is possible to design each laser102A and 104A so that zones one laser are paired to zones of the otherlaser. The pairing can be one-to-one, or one-to-many. The case of aone-to-one pairing is straightforward (FIG. 12): each laser 102A and104A has two or more zones in the first reflectors 106A and 108A wherethe overlap condition is satisfied and it is satisfied using a specificzone of laser 102A (R_(1a)) and a specific zone of laser 104A (R_(2a))such that neither R_(1a) or R_(2a) are paired with other zones. In someembodiments, as seen in FIG. 12, the predetermined frequency intervaldefines a first interval region and a second interval region, the firstand second wavelength-frequency relationships being such that the firstand second wavelengths are respectively monotonous functions of theadjustable frequency over each of the first and second interval regions.

In other embodiments, one or more of the first and second lasers 102 and104 is able to emit more than one wavelength in each light pulse.

The case of one-to-many pairing (FIG. 13, system 100B) is accomplishedby matching the different harmonics of a zone (n·R_(1a)) in the firstreflector 1068 of one laser 1028 with different zones in the firstreflector 1088 of the other laser 104B (R_(2a), R_(2b)) such that(N·R_(1a)=R_(2a) and M·R_(1a)=R_(2b)) where N and M are integer. In bothcases, we can go from one pairing to another simply by changing therepetition rate of the lasers 102B and 104B. The DTAMLs can besynchronized over multiple zones that either have the same repetitionrates (one-to-one pairing) or are harmonic of each others (one-to-manypairing).

If we create parallel paths in one laser or both lasers 102C and 104C,as in system 100C shown in FIG. 14, such that different zones have thesame repetition rate, a wavelength will be emitted for each parallelzone. If both lasers 102C and 104C have such parallel zones we can matcha set of (one or more) wavelengths from one laser to a set of (one ormore) wavelengths of the other lasers. Note that the parallel branchescan be combined with the pairing of different zones, as in the system100C. In these embodiments, the lasers 102C and 104C have a structuresimilar to the structure of laser 10H described hereinabove.

It should be noted that having two lasers, as opposed to one laseremitting many wavelengths, creates difficulty in pulse synchronization,which are solved by the present invention, while allowing advantageouslyto compensate for the group delay outside of the lasers. Thischaracteristic is very important in many applications. In addition,using a single laser to emit pulses at multiple wavelengths can beproblematic with some gain media. The use of two lasers solves theseproblems.

While synchronization of two lasers is described hereinabove, more thantwo lasers can also by synchronized in a similar manner.

Although the present invention has been described hereinabove by way ofpreferred embodiments thereof, it can be modified, without departingfrom the spirit and nature of the subject invention as defined in theappended claims.

1. A synchronized laser system for illuminating a sample with first andsecond laser light pulses having respectively first and secondwavelengths, said system comprising: a trigger, said trigger beingoperative to issue first and second trigger signals, said first andsecond trigger signals being periodic and emitted at a common adjustablefrequency with a predetermined delay therebetween, said adjustablefrequency being included in a predetermined frequency interval; a firsttunable mode-locked laser operative for emitting said first laser lightpulses in response to receiving a first train of said first triggersignals, said first wavelength of said first laser light pulses beingdependent on said adjustable frequency in accordance with a firstwavelength-frequency relationship, said first tunable mode-locked laserbeing operative over a first repetition rate range part of saidpredetermined frequency interval to produce said first laser light withsaid first wavelength within a first laser tuning range; a secondtunable mode-locked laser operative for emitting said second laser lightpulses in response to receiving a second train of said second triggersignals, said second wavelength of said second laser light pulses beingdependent on said adjustable frequency in accordance with a secondwavelength-frequency relationship, said second tunable mode-locked laserbeing operative over a second repetition rate range part of saidpredetermined frequency interval to produce said second laser light withsaid second wavelength within a second laser tuning range; wherein saidpredetermined delay is such that said first and second laser lightpulses are emitted so as to arrive substantially simultaneously in saidsample; and said first and second wavelength-frequency relationships areselected to result in a predetermined relationship between said firstand second wavelengths at each adjustable frequency from saidpredetermined frequency interval at which said first and secondrepetition rate ranges overlap.
 2. A system as defined in claim 1,wherein said first and second wavelength-frequency relationships aresuch that said second wavelength varies less as a function of saidadjustable frequency than said first wavelength over said predeterminedfrequency interval so that over said predetermined frequency interval,said first wavelength varies more than said second wavelength.
 3. Asystem as defined in claim 2, wherein said second wavelength varies atleast 100 times more slowly than said first wavelength as as function ofsaid adjustable frequency.
 4. A system as defined in claim 2, whereinsaid sample defines an interaction bandwidth of interest includingwavelengths over which a predetermined light-matter interaction occurs,said second laser tuning range being within said interaction bandwidth.5. A system as defined in claim 4, wherein said sample includes anon-linear material, said predetermined light-matter interactionincluding a non-linear light-matter interaction occurring in saidinteraction bandwidth of interest.
 6. A system as defined in claim 5,wherein said non-linear material is a frequency doubling material.
 7. Asystem as defined in claim 6, wherein said frequency doubling materialis a Lithium Niobate (LiNbO₃) crystal, a Barium Borate crystal (BaB₂O₄)or a Potassium Titanyl Phosphate crystal (KTiOPO₄).
 8. A system asdefined in claim 2, further comprising a non-linear material insertedbetween said second tunable mode-locked laser and said sample.
 9. Asystem as defined in claim 8, wherein said non-linear material is afrequency-doubling material operative for producing light having a thirdwavelength that is half the first wavelength when illuminated with saidfirst laser light pulses.
 10. A system as defined in claim 2, whereinsaid first repetition rate range is entirely included in said secondrepetition rate range.
 11. A system as defined in claim 2, wherein saidfirst and second wavelength-frequency relationships are such that saidfirst and second wavelengths are respectively a first and a secondmonotonous function of said adjustable frequency.
 12. A system asdefined in claim 1, wherein said predetermined frequency intervaldefines a first interval region and a second interval region, said firstand second wavelength-frequency relationships being such that said firstand second wavelengths are respectively monotonous functions of saidadjustable frequency over each of said first and second intervalregions.
 13. A system as defined in claim 1, wherein said first tunablemode-locked laser is operative for emitting third laser light pulses inresponse to receiving said first train of said first trigger signals, athird wavelength of said third laser light pulses being dependent onsaid adjustable frequency in accordance with a thirdwavelength-frequency relationship, said first tunable mode-locked laserbeing operative over said first repetition rate range to also producesaid third laser light with said third wavelength within a third lasertuning range.
 14. A system as defined in claim 12, wherein said secondtunable mode-locked laser is operative for emitting fourth laser lightpulses in response to receiving said second train of said second triggersignals, a fourth wavelength of said fourth laser light pulses beingdependent on said adjustable frequency in accordance with a fourthwavelength-frequency relationship, said second tunable mode-locked laserbeing operative over said second repetition rate range to also producesaid fourth laser light with said fourth wavelength within a fourthlaser tuning range.
 15. A system as defined in claim 1, wherein outsideof said first and second repetition rate ranges, said first and secondtunable mode-locked lasers and are inoperational to produce respectivelysaid first and second laser light pulses.